Which of the following elements has the highest metallic character?

Metallic character refers to the tendency to lose electrons. Li, Na, K and Rb are the members of group 1. As we move down the group from Li to Rb, the atomic size increases because the number of shells increases. Due to this increased atomic size, the effective nuclear charge experienced by electrons present in the outermost shell decreases. As a result, it becomes easier for an element to lose its electron, i.e. on moving down the group, metallic character increases. Therefore, Li is the least metallic, whereas Rb is the most metallic from all the given elements.